When manufacturing methanol and ammonia, a reformed gas obtained by reforming the natural gas or the like in the reformer is used (for example, see Patent Literatures 1 and 2). When the natural gas or the like is reformed in the reformer to convert the natural gas to the reformed gas, before supplying the natural gas to the reformer, a part of the natural gas is extracted and used as fuel of the reformer, and the natural gas supplied to the reformer is reformed.